The objectives of this research are to provide general information on the process of enzymatic hydroxylation and the biosynthetic pathway to the pteridine ring and to investigate the functions of pteridines in addition to their actions as hydroxylase cofactors. Recently it has been shown that the action of phenylalanine hydroxylase is regulated in a number of ways including regulation by further products of tyrosine metabolism. The molecular weight of the enzyme has been determined and many of its properties elucidated. A new activation procedure has been developed which hopefully will aid in further purification of this important enzyme. Dihydropteridine reductase in Pseudomonas has been purified and its action characterized, and the first enzyme in the pathway to pteridine biosynthesis has been studied in detail.